When immune complexes bind the C1q subcomponent of C1, the bound C1q has two important activities: 1) providing a binding site for C1r2s2, the catalytic unit of C1, which activates the rest of the complement cascade; and 2) stimulating cells bearing the C1q receptor. The overall objectives of this proposal are to identify and characterize the natural inhibitors of C1, which are responsible for the specific and restricted functional activity of C1 in vivo. In particular, we are focusing on two inhibitors, Factor J and heparan sulfate, which are potentially able to control the recycling of bound C1q as an activator of Clr2s2, and as a stimulus to cells bearing C1q receptors. Factor J was initially identified and isolated from human urine and serum in the Principal Investigator's laboratory. It is a heavily glycosylated, basic protein, comprised of 200,000, 18,000, and 5,900 subunits, which is expressed on some circulating cells. Factor J binds to C1q and is a potent non-competitive inhibitor of the assembly of macromolecular C1 from the C1q and C1r2s2 subcomponents. It is structurally and functionally distinct from other known inhibitors of C1. The protein sequence of Factor J will be determined by Edman degradation or mass spectrometry using proteolytic fragments of deglycosylated protein. By producing high titer polyclonal and monoclonal antibodies it will be possible to define the functional and antigenic domains of the protein, characterize the expression of Factor J antigen in various tissues, and perform biosynthetic studies.